JERUSALEM, May 5 (Jeffrey Heller/Reuters) - Iran was squarely in Israel's sights when it sent its planes to hit targets in Syria, waging a war-within-a-war that showed a readiness to strike out alone if its red lines were crossed.

Allegations of Syrian government forces using chemical weapons have grabbed headlines and driven new calls for U.S. President Barack Obama to intervene in Syria's civil war.

But when it took military action over the weekend while Washington stayed on the sidelines, Israel was homing in on targets with strategic significance for its own possible war with Iran rather than for Syria's internal fighting.

In both Israeli attacks, on Friday and Sunday, long-range, Iranian-supplied missiles destined for Lebanon's Hezbollah guerrilla group were hit, Israeli and Western sources said.

Such weapons, along with what Israel believes to be a Hezbollah arsenal of about 60,000 other rockets, could pose a significant threat to Israeli cities in any future conflict.

Although the militant group could opt to strike any time, Israeli officials are particularly concerned about Hezbollah missile barrages as proxy retaliation should Israel carry out long-threatened attacks against Iranian nuclear facilities.

We have very clear guidelines. We will not let game-changing weaponry reach the hands of Hezbollah. We will do whatever is necessary to stop that, said Ofer Shelah of the Yesh Atid party, a member of Israel's governing coalition.

Shelah, who also sits on parliament's foreign affairs and defense committee, was referring to a red line Prime Minister Benjamin Netanyahu has set on the Syrian conflict. Beyond that, it is a murky situation, Shelah added, pointing to Israeli ambivalence over the fate of President Bashar al-Assad.

NO BLUFF

A red line that Netanyahu famously drew for Iran's uranium enrichment program, in a cartoon bomb produced during a speech at the United Nations last September, has seemed more flexible.

Last week, Netanyahu, who forecast Iran would cross the line in mid-2013, said it was still short of that mark. This raised further doubts over whether Israel would opt, against long-standing U.S. advice, to launch a unilateral strike against what it believes is an Iranian bid to develop nuclear weapons.

For Israel, the threshold will be reached once Iran, which denies seeking atomic arms, will have amassed enough uranium at 20-percent fissile purity that could quickly be used to fuel just one nuclear bomb.

Amos Yadlin, a former chief of Israeli military intelligence, said Israel's strikes in Syria sent a strong message to Iran that Netanyahu was not bluffing.

Iran is testing the determination both of Israel and the U.S. regarding red lines, and what it sees in Syria is that at least some of the players take the red lines seriously, Yadlin told Army Radio, appearing to take a dig at Washington's inaction so far.

Obama, in an interview on Saturday with the Spanish-language network Telemundo, said the Israelis justifiably have to guard against the transfer of advanced weaponry to Hezbollah.

Uzi Rubin, an Israeli missile expert and former defense official, said the Fateh-110 missile reportedly targeted in the Israeli strikes is better than the Scud, it has a half-ton warhead. It may also be more accurate than other rockets.

CALM EXTERIOR

Signalling that Israel was not overly concerned about possible retaliation by Assad's forces or Hezbollah for the air strikes, Netanyahu planned to leave later in the day for a five-day visit to China, Israeli officials said.

But two of Israel's Iron Dome anti-missile batteries were deployed near the northern fronts with Syria and Lebanon - Hezbollah's homeland. Netanyahu also convened a last-minute meeting of his security cabinet before his planned departure.

Our estimate is that Assad will not respond to this by attacking Israel, one Israeli official said. He knows that doing so would draw counter strikes that will seriously impair his military capabilities and therefore potentially allow the rebels to even their odds against him.

Technically, Israel is still at war with Syria following the 1973 Yom Kippur war, but in reality the northern borderlands have been relatively quiet for the past four decades.

The air strikes on Syria caused no immediate political fallout in Israel, where containment of Hezbollah - which fired more than 4,000 rockets into the country during a war in 2006 - is a consensus issue that unites people across party lines.

Illustrating concern over Hezbollah, Amos Gilad, a senior Defense Ministry official, said in a lecture in the southern city of Beersheba on Saturday that the group was keen to take weapons systems (in Syria), like rockets that can reach, say, all the way here.

Hezbollah portrays itself as arming against aggression by Israel, which occupied southern Lebanon for two decades until 2000 and with which Lebanon still has territorial disputes.

Netanyahu has given no public sign of planning a military intervention in Syria, despite the occasional spillover of fire from the conflict into the Israeli-occupied Golan Heights.

In the Israeli attack on Friday, Israeli planes did not enter Syrian air space, U.S. officials said, apparently firing at their targets from neighboring Lebanon to avoid any direct confrontation with Assad's military. It was not immediately clear whether the same held true for Sunday's strike.

Taking sides in Syria is problematic: Israelis believe one in 10 of the rebels fighting Assad, who has followed his father in keeping the peace on the Golan since 1974, is a jihadist who might turn his gun on them once the Syrian leader were gone.

Tzachi Hanegbi, a Netanyahu confidant and a legislator from his right-wing Likud party, told Army Radio on Sunday Israel had no position on whether Assad should stay or go, adding that the government did not want to bet on the wrong horse.

What we want, he said, Is to ensure that within the Syrian chaos we will not see Hezbollah growing stronger, in a way that will motivate it to act against us and draw us into a conflict in which we will suffer grave casualties. (Additional reporting by Maayan Lubell, Crispian Balmer and Dan Williams; Editing by Alastair Macdonald)